Fibre optic distributed sensing

This application describes methods and apparatus for distributed fiber optic sensing that reduce or compensate for the effects of laser phase noise. The method involves repeatedly interrogating an optical fiber (101), where each interrogation involves launching at least one interrogating pulse into the optical fiber; detecting optical radiation which is backscattered from within the fiber, and forming a measurement signal from a plurality of sensing portions of the fiber. The method involves forming a laser noise template based on the measurement signals within a frequency band of interest from a first set of said sensing portions; and applying a correction to the measurement signals from said plurality of sensing portions based on said laser noise template. The first set of sensing portions may be temperature and/or strain stable portion from an isolated section of fiber (301, 303) or may comprise a selected set of sensing portions, which may include substantially all the sensing portions, which are averaged to form the laser phase noise template. For a phase based DAS type sensor the laser phase noise template may simply be subtracted from the measurement signals. For an intensity based sensor it may be necessary to polarity align and normalize the signals of the first set before averaging and then determine a gain-time profile for each sensing portion prior to applying the correction.